1. Field of the Invention
The present invention relates to a nitride-based semiconductor element and a method of forming a nitride-based semiconductor, and more specifically, it relates to a nitride-based semiconductor element containing a nitride-based semiconductor formed by epitaxial lateral overgrowth and a method of forming a nitride-based semiconductor.
2. Description of the Prior Art
A technique of growing a nitride-based semiconductor on an underlayer is known in general. For example, a crystal of GaN, which is one of nitride-based semiconductors lattice-matching with only a small number of types of substrates, is grown on a substrate such as a sapphire substrate. In relation to this, generally known is a technique of inserting a buffer layer grown at a low temperature between the substrate and a GaN layer for growing GaN in excellent crystallinity with a small number of crystal defects.
Even if the aforementioned low-temperature buffer layer is employed, however, the density of reducible defects is limited and it is difficult to reduce the number of dislocations. To this end, a technique of reducing the number of dislocations by epitaxial lateral overgrowth (ELOG) of GaN is proposed in general. This epitaxial lateral overgrowth is disclosed in Journal of Oyo Denshi Bussei Bunkakai, Vol. 4 (1998), pp. 53 to 58 and 210 to 215, for example.
FIG. 29 is a sectional view for illustrating a conventional method of forming a nitride-based semiconductor by epitaxial lateral overgrowth. Referring to FIG. 29, a low-temperature buffer layer 102 is first formed on a sapphire substrate 101, and thereafter a GaN layer 103 for serving as an underlayer is grown on the low-temperature buffer layer 102 in the conventional method of forming a nitride-based semiconductor by epitaxial lateral overgrowth.
Then, striped (elongated) mask layers 104 of SiO2 or the like are formed on prescribed regions of the GaN layer 103. The mask layers 104 are employed as selective growth masks for epitaxially laterally overgrowing a GaN layer 105 from the GaN layer 103 serving as an underlayer, so that the GaN layer 105 is vertically (upwardly) grown from exposed portions of the GaN layer 103 and thereafter laterally grown on the mask layers 104. Dislocations extending in the c-axis direction are laterally bent due to this lateral overgrowth, not to reach a portion around the upper surface of the GaN layer 105. Thus, the number of dislocations reaching the flat upper surface of the finally formed GaN layer 105 is remarkably reduced as compared with the GaN layer 103 forming the underlayer.
In the conventional method of forming a nitride-based semiconductor by epitaxial lateral overgrowth shown in FIG. 29, however, the c-axis of the overgrowth region of GaN layer 105 is disadvantageously inclined from the normal direction of the substrate although the number of dislocations resulting from epitaxial lateral overgrowth can be reduced in the GaN layer 105. In other words, the growth layer of the GaN layer 105 laterally grown on the mask layers 104 is strained due to stress applied thereto. Therefore, the c-axis perpendicular to the sapphire substrate 101 is disadvantageously inclined by about 2° at the maximum as shown by arrows in FIG. 29. When the c-axis is displaced, crystallinity is deteriorated to result in inferior element characteristics.
In order to suppress such inclination of the c-axis, therefore, a method shown in FIG. 30 is proposed in general. Referring to FIG. 30, a GaN layer 113 serving as an underlayer is formed on a sapphire substrate 111 through a low-temperature buffer layer 112 in this proposed method. Recess portions are formed on the surface of the GaN layer 113 for thereafter forming mask layers 114 having recess portions 114a of SiO2 or the like in recess portions 113a of the surface. The mask layers 114 are employed as selective growth masks for epitaxially laterally overgrowing a GaN layer 115 on projection portions of the GaN layer 113 serving as an underlayer. In this case, voids 120 are defined between the mask layers 114 and the epitaxially laterally overgrown GaN layer 115, thereby reducing the contact areas between the GaN layer 115 and the mask layers 114 when the GaN layer 115 is laterally grown on the mask layers 114. Thus, stress is hardly applied to the GaN layer 115 laterally grown on the mask layers 114, and hence strain of the GaN layer 115 is relaxed. Consequently, inclination of the c-axis can be relaxed as compared with the prior art shown in FIG. 29, as shown by arrows in FIG. 30.
In the conventional proposed method shown in FIG. 30, however, it is necessary to etch the GaN layer 113 serving as an underlayer for forming the recess portions thereof. In this case, a long time is required for etching the GaN layer 113, disadvantageously leading to a long process time.
In the conventional proposed method shown in FIG. 30, most dislocations 116 are bent in intermediate portions during lateral growth of the GaN layer 115 not to reach the surface, as shown in FIG. 31. However, some of dislocations 116 are not bent but reach the surface as such. In the conventional proposed method, therefore, it is difficult to further reduce the number of dislocations.